September finds the streets filled with music, art and people celebrating the last few weeks of summer before the whole big party moves indoors

The Affordable Art Fair 2011
With the goal of shaking up the dusty model of art as an elite pastime in mind, the Affordable Art Fair Fall 2011 will showcase new artists, galleries, programming, and installations and help make contemporary art accessible to all collectors.
September 21-25, see site for hours
7 West 34th Street (near Fifth Avenue)
Tickets: $17-$65 (private preview)

85th Annual Feast of San Gennaro
The Feast of San Gennaro is New York City’s longest-running, biggest and most revered religious outdoor festival. Every year, the streets of historic Little Italy host the 11-day event featuring religious processions and colorful parades, free musical entertainment every day, a wide variety of ethnic food delicacies, a world-famous cannoli-eating competition and more.
September 15-25
Mulberry Street, Grand Street and Hester Street between Canal and Houston Streets

Boardwalk Empire Season Two Premiere Viewing Party
The Madison Club Lounge at the Roosevelt Hotel celebrates the premiere of the second season of the HBO mobster hit in speakeasy style. Step back in time, wear your finest Boardwalk Empire looks and capture the evening with classic old-fashioned photo ops and vintage cocktails. Entry to the event is free (with a cocktail promotion between 8-10 p.m.) but there will be a food menu available until midnight. Prizes will be awarded for the best-dressed guests (via Guest of a Guest).
Sunday, September 25, 7 p.m. - 11 p.m.
The Madison Club Lounge, 45 East 45th Street at The Roosevelt Hotel

Climate Week NYC
Climate Week NYC promotes climate-focused events like the DJ Spooky Art of Climate Science launch at New York Academy of Sciences which explores climate change through graphic art and music and a full calendar of educational and fun opportunities for the whole family to discover all things green and clean.
September 19-26
Locations vary, see site and schedule for info

Live Music Round-Up
Celebrated Elephant 6 Collective The Olivia Tremor Control make a rare appearance at Le Poisson Rouge; endlessly quotable moody balladeer Stephen Malkmus (and the Jicks) perform at Webster Hall; Indie top-10-list darlings Fleet Foxes will be playing at the Williamsburg Waterfront along with darlings-of-years-past Walkmen; Michael Gira’s seminal ’80s project, the dark and dangerous Swans will be at Music Hall of Williamsburg.